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The patterns of spatial and temporal variability in CO2 and CH4 emission from reservoirs are still poorly studied, especially in tropical regions where hydropower is growing. We performed spatially resolved measurements of dissolved CO2 and CH4 surface water concentrations and their gas‐exchange coefficients (k) to compute diffusive carbon flux from four contrasting tropical reservoirs across Brazil during different hydrological seasons. We used an online equilibration system to measure dissolved CO2 and CH4 concentrations; we estimated k from floating chamber deployments in conjunction with discrete CO2 and CH4 water concentration measurements. Diffusive CO2 emissions were higher during dry season than during rainy season, whereas there were no consistent seasonal patterns for diffusive CH4 emissions. Our results reveal that the magnitude and the spatial within‐reservoir patterns of diffusive CO2 and CH4 flux varied strongly among hydrological seasons. River inflow areas were often characterized by high seasonality in diffusive flux. Areas close to the dam generally showed low seasonal variability in diffusive CH4 flux but high variability in CO2 flux. Overall, we found that reservoir areas exhibiting highest emission rates ("hotspots") shifted substantially across hydrological seasons. Estimates of total diffusive carbon emission from the reservoir surfaces differed between hydrological seasons by a factor up to 7 in Chapéu D'Úvas, up to 13 in Curuá‐Una, up to 4 in Furnas, and up to 1.8 in Funil, indicating that spatially resolved measurements of CO2 and CH4 concentrations and k need to be performed at different hydrological seasons in order to constrain annual diffusive carbon emission. Plain Language Summary: Reservoirs are key for flood control, water supply, and hydropower generation. However, reservoirs are usually not carbon neutral. Studies worldwide point to reservoirs as important net sources of anthropogenic carbon emission to the atmosphere. Carbon emission from reservoirs derives from the decomposition of organic matter. Although carbon emission from reservoirs has been increasingly studied over the past two decades, most studies do not sufficiently describe emissions across space and time. Our study applies highly resolved spatial coverage of dissolved surface water concentrations and gas‐exchange coefficients of CO2 and CH4 to compute rates of CO2 and CH4 diffusion to the atmosphere across distinct hydrological seasons in four contrasting tropical reservoirs. We found that emissions varied substantially over both space and time. More specifically, we found that reservoir areas exhibiting highest emission rates ("hotspots") shifted substantially between dry and rainy seasons. Overlooking the spatial within‐reservoir variability across seasons may result in serious underestimation or overestimation of total diffusive carbon emission from reservoirs, depending on the time and space that studies focus their sampling on. Our work may support scientists in adopting more comprehensive sampling strategies relevant for better constrained upscaling, and, consequently, support informed policy decisions and management actions. Key Points: Reservoir regions displaying highest emission rates shifted substantially between sampling occasions at hydrologically different seasonsRiver inflow areas were often characterized by high seasonality in diffusive carbon fluxSpatially resolved measurements need to be conducted at different hydrological seasons in order to constrain diffusive emission [ABSTRACT FROM AUTHOR]

While warming and eutrophication have increased the frequency and magnitude of harmful cyanobacterial blooms globally, the scenario for many eutrophic tropical freshwaters is a perennial year-round bloom. Yet, the drivers of persistent blooms are less understood when conditions such as light, temperature, and nutrients favor cyanobacteria growth year-round, and especially in regions facing recurrent periods of drought. In order to understand the drivers of cyanobacteria dominance, we assessed the abiotic conditions related to the abundance and dominance of the two dominant bloom-forming genera Raphidiopsis and Microcystis, in six shallow, man-made lakes located in the semiarid Northeastern region of Brazil during a prolonged regional drought. Lower water level corresponded to increased phosphorous and nitrogen concentration and, consequently, phytoplankton biomass. Cyanobacterial biomass was also proportional to phosphorus concentrations during year-round blooms. Yet, the two dominant cyanobacterial genera, Raphidiopsis and Microcystis, seldom co-occurred temporally and the switch between them was driven by water transparency. Our results illustrate the effects of drought induced water level reductions on the biomass and composition of cyanobacterial blooms in tropical shallow man-made lakes. Given the ideal year-round conditions (i.e., high light and temperature), droughts may be expected to intensify the risk and multitude of problems associated with eutrophication. [ABSTRACT FROM AUTHOR]

The discussers wish to thank the authors of the original paper for investigating the comparing accuracy of artificial intelligence techniques trained to predict chlorophyll-a in US lakes. In the original paper (Luo et al., Environ Sci Pollut Res 26: 30524–30532, 2019), four data-driven models were established to estimate the chlorophyll-a (CHLA) values in natural and man-made lakes. Three of these models are adaptive neuro-fuzzy inference system (ANFIS)-based, while one is (artificial neural network) ANN-based. The authors used total phosphorus (TP), total nitrogen (TN), turbidity (TB), and the Secchi depth (SD) as independent variables in order to predict CHLA. They stated that ANFIS with subtractive clustering method (ANFIS_SC) models and multilayer perceptron neural network (MLPNN) models gives higher accuracy in the prediction of CHLA values for natural lakes and man-made lakes, respectively. In this letter, some of the missing points in the original publication, which is important for the estimation and comparison of CHLA values in two different lake sets that differ according to the type of formation, are highlighted. In addition, several points are mentioned in order to make these points more clarified for potential readers. [ABSTRACT FROM AUTHOR]

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Abstract Aim The aim of this study was to analyze the water quality of a tropical, semi-arid reservoir after a reflooding. In terms of impact on water quality after a drought event, it is expected that there will be improvements with the reflooding. Less algal biomass, increased water transparency, decreased turbidity and low nutrient concentration. Methods This study was performed in a tropical, semi-arid man-made lake (Dourado Reservoir), during an extended drought period. This study consisted of a comparison of three distinct periods determined by water accumulation. The limnological variables, including water transparency, turbidity, electrical conductivity, pH, total phosphorus, soluble reactive phosphorus, and chlorophyll-a were analyzed. A principal component analysis (PCA) was also performed to verify the patterns of the variables in relation to the sample units in the studied periods. Results After water renewal, there was an expressive reduction in chlorophyll-a. Electrical conductivity, pH, and turbidity variables also reduced after the reflooding, indicating an improvement in water quality. There was no reduction in total phosphorus and soluble reactive phosphorus after the reflooding compared to the previous periods. Conclusions The significant reduction in algal biomass after reflooding in Dourado indicates water quality improvement in terms of eutrophication due to the change of the trophic state from eutrophic to mesotrophic.

Chlorophyll-a (CHLA) is a key indicator to represent eutrophication status in lakes. In this study, CHLA, total phosphorus (TP), total nitrogen (TN), turbidity (TB), and Secchi depth (SD) collected by the United States Environmental Protection Agency for the National Lakes Assessment in the continental USA were analyzed. Statistical analysis showed that water quality variables in natural lakes have strong patterns of autocorrelations than man-made lakes, indicating the perturbation of anthropogenic stresses on man-made lake ecosystems. Meanwhile, adaptive neuro-fuzzy inference systems (ANFIS) with fuzzy c-mean–clustering algorithm (ANFIS_FC), ANFIS with grid partition method (ANFIS_GP), and ANFIS with subtractive clustering method (ANFIS_SC) were implemented to model CHLA in lakes, and modeling results were compared with the multilayer perceptron neural network models (MLPNN). Results showed that ANFIS_FC models outperformed other models for natural lakes, while for man-made lakes, MLPNN models performed the best. ANFIS_GP models have the lowest accuracies in general. The results indicated that ANFIS models can be screening tools for an overall estimation of CHLA levels of lakes in large scales, especially for natural lakes. [ABSTRACT FROM AUTHOR]

The purpose of this paper was to assess the effect of the diversion of wastewater on the trophic status of Lake Bidighinzu, a hypertrophic man-made lake in Northern Sardinia, used as a drinking water reservoir. There have been problems with potabilization since the early years of the diversion operation, particularly in the summer-autumn period. Data available (August 1978, February 1979 and March 1985) before the reservoir (1987) were compared with those collected during a study carried out in the annual cycle immediately after (1988-1989) and after some years (1994 and 1996-1997). The study examined the dynamics of temperature, main nutrients (total phosphorus, nitrate and ammonia), chlorophyll-a and phytoplanktonic biomass. No particular variation in the water nutrient availability emerged from the comparison between these two situations - especially for total phosphorus, whose annual mean concentrations were similar in the two annual cycles (386 mg P m-3 in 1988-1989 and 305 mg P m-3 in 1996-1997). Chlorophyll-a and biomass were high during each period of study (annual mean values were 17 mg m-3and 3.7 mg l-1 in 1988-1989 and 11 mg m-3and 4.6 mg l-1 in 1996-1997). However, peaks were never higher than values recorded in August 1978 (112 mg m-3and 133 mg l-1) due to an extraordinary bloom of Ceratium hirundinella (O.F. Müller). Species composition of phytoplankton was typical of highly trophic conditions and was frequently characterised by the presence of Cyanophyceae and Bacillariophyceae. Results demonstrated that, ten years after construction of the by-pass, the lake had shown no improvement in its trophic status. This might depend on many factors, among which the most likely might be the non-use or partial use of the diversion system, which allows the sewage to continue to flow into the lake and keep its trophic status almost unchanged.

Abstract Hydroelectric reservoirs are novel ecosystems that provide a variety of important services. To manage these ecosystems and their fish populations effectively, we need to develop conceptual frameworks for predicting their short‐ and long‐term responses. To advance this goal, we revisited and tested the “trophic surge hypothesis, TSH.” The TSH has been widely cited in the literature, but has not been empirically tested across numerous reservoirs. The TSH suggests that fish populations should show a hump‐shaped pattern (i.e., the non‐equilibrium phase) after river impoundment. As such, we assembled 40 recruitment and 109 adult fish abundance time series from 19 species distributed across seven reservoirs from temperate and boreal regions, and applied curve fitting analyses and model selection. We found that the hump‐shaped pattern was the predominant pattern across individual time series, providing moderate support for the TSH. Fish recruitment increased substantially during reservoir filling and was followed by an increase in adult fish lagging 3–4 years behind. The non‐equilibrium phase was transient and lasted roughly eight years for recruits, whereas it could be much longer for adults. When time series were combined across regions and sites, the support for the TSH was weaker. However, we observed significant variability in the duration, timing, and magnitude of the surge across individual time series and found that the total flooded area was the most influential predictor to explain this variability. In conclusion, the TSH and related metrics can be a useful and general predictive framework to understand how fish populations may respond to impoundment. In particular, long‐term management recommendations could be short‐sighted if formulated before convincing evidence has emerged to show that the reservoir reached its new trophic equilibrium.

A probabilistic sample of lakes in the 48 coterminous US lakes was made by the United States Environmental Protection Agency in the 2007 National Lakes Assessment. Because of the statistical design, the results of our analyses of Secchi depths (SD) apply to a population of 45,265 lakes. We found statistically significant differences in mean Secchi depths between natural (1.57 m) and man-made lakes (1.18 m). The most important variable correlated with SD was turbidity, an optical measure related to suspended particles in the water column. For most lakes, chlorophyll a was highly correlated with both turbidity and SD, but several lakes had more turbidity and lower SD than expected based on chlorophyll a alone, indicating that non-algal suspended solids were an important factor. On an ecoregion basis, the non-algal suspended solids in the lake waters were related to the average levels of suspended solids in streams located in that ecoregion, and the non-algal suspended solids were more important in man-made than natural lakes. Phosphorus and nitrogen were directly correlated with chlorophyll a and turbidity and inversely correlated with SD. Based on diatom-inferred Secchi depths for the tops and bottoms of sediment cores from lakes in Ecoregions VIII and VII (excluding lakes in Minnesota) representing 40% of the natural lakes in the US, there has been no decrease in water transparency in that population of lakes in the past 70 or more years when the US population increased by 134%. We do not have information to determine if the other 60% of lakes have or have not changed. [ABSTRACT FROM AUTHOR]

Hydroelectric reservoirs are novel ecosystems that provide a variety of important services. To manage these ecosystems and their fish populations effectively, we need to develop conceptual frameworks for predicting their short- and long-term responses. To advance this goal, we revisited and tested the 'trophic surge hypothesis, TSH.' The TSH has been widely cited in the literature, but has not been empirically tested across numerous reservoirs. The TSH suggests that fish populations should show a hump-shaped pattern (i.e., the non-equilibrium phase) after river impoundment. As such, we assembled 40 recruitment and 109 adult fish abundance time series from 19 species distributed across seven reservoirs from temperate and boreal regions, and applied curve fitting analyses and model selection. We found that the hump-shaped pattern was the predominant pattern across individual time series, providing moderate support for the TSH. Fish recruitment increased substantially during reservoir filling and was followed by an increase in adult fish lagging 3-4 years behind. The non-equilibrium phase was transient and lasted roughly eight years for recruits, whereas it could be much longer for adults. When time series were combined across regions and sites, the support for the TSH was weaker. However, we observed significant variability in the duration, timing, and magnitude of the surge across individual time series and found that the total flooded area was the most influential predictor to explain this variability. In conclusion, the TSH and related metrics can be a useful and general predictive framework to understand how fish populations may respond to impoundment. In particular, long-term management recommendations could be short-sighted if formulated before convincing evidence has emerged to show that the reservoir reached its new trophic equilibrium. [ABSTRACT FROM AUTHOR]

The construction of ponds for irrigation has proliferated during last decades in Mediterranean regions. If properly constructed and managed, ponds may simultaneously meet conservation and agricultural objectives: the preservation of certain macrophytes might improve water quality for irrigation purposes. However, the effects of management are often neglected when analyzing the main drivers of organisms community structure. Here we investigated the effect of environmental variables, construction type and management practices on the occurrence of submerged macrophytes (vascular plants, charophytes, filamentous algae). Total submerged macrophyte coverage and richness did not vary with pond type, but the presence of submerged macrophytes was significantly higher in embankment ponds. Artificial ponds showed the highest charophyte richness but lowest vascular plant occurrence. Total submerged macrophyte occurrence mainly correlated with conductivity and littoral vegetation, whilst the relative importance of environmental factors was higher than variables related to pond type and management. However, embankment ponds showed the highest richness of vascular plants and filamentous algae, and artificial ponds contributed to the regional pool of charophyte species. Thus, construction pond type and management levels might be relevant to understand submerged macrophytes occurrence in man-made ponds, since the three pond types contributed to the local and regional species pool. [ABSTRACT FROM AUTHOR]

Provides a description of the limnochemical conditions of the northern portion of the Volta lake in Ghana. Assessment of the long-term changes in water quality of the man-made lake; Factors that led to changing limnochemical conditions observed in the lake; Impact of seasonal changes on water temperature; Transparency of the Volta lake measured in the study.